Silicon carbide (SiC) has a wide band gap and high electric field strength and thus is used as a material of high-power semiconductor devices having high breakdown voltages. For example, Schottky barrier diodes (SBD) which are made of SiC are used for high speed switching. However, in a case of a SBD made of SiC, if a forward current such as a surge current significantly exceeding the rating of the SBD is applied, a forward voltage increases due to substrate resistance, thereby causing an element breakdown. For this reason, various technologies for improving surge current withstand are used.
For example, there has been proposed a structure including a first electrode layer configuring a Schottky junction and a second electrode layer formed separately from the first electrode layer so as to form an ohmic junction with a guard ring disposed around the outer periphery of a SBD region (see Japanese Patent Application Laid-Open No. 2003-258271 for instance). According to this structure, in a case where a forward voltage rises, minority carriers are injected from a PN junction formed by the guard ring which is a p-type region and an n-type drift region into the drift region. The minority carriers diffuse into the drift region positioned below the Schottky junction, whereby conductivity modulation occurs. As a result, the series resistance of the drift region decreases, whereby the forward voltage is suppressed from rising. If the PN junction of the guard ring is operated as a PN diode as described above, it is possible to suppress a breakdown due to a surge current.
However, according to the above described method, in a case where the guard ring and the second electrode layer do not form an ohmic junction, a very high voltage is required for operating as a PN diode. For this reason, the PN junction may not operate as a PN diode until the voltage becomes almost equal to a voltage at which a breakdown may occurs due to the surge current occurs. In this case, forward surge current withstand does not improve. For this reason, in the method disclosed in Japanese Patent Application Laid-Open No. 2003-258271, the second electrode layer is formed of a metal, such as aluminum (Al), gold (Au), platinum (Pt), or an alloy thereof, so as to form an ohmic junction with the guard ring which is an p-type region, and the second electrode layer is formed separately from the first electrode layer which forms a Schottky junction.